1. Technical field
The present invention relates to a method for separating caesium, strontium and transuranium elements present in sodium waste, and to a device for implementing this method.
The present invention may, for example, be applied to the treatment of radioactive sodium waste derived from nuclear waste treatment.
In particular it provides the possibility of limiting the inventory of materials involved to separate or isolate the above-cited elements from the waste, of controlling and managing chemical reaction time, of conducting separation as a continuous process and of guaranteeing the safety of personnel and equipment throughout the separating process.
It also provides for optimum adjustment of the quantities of reagents added, and limits undesired chemical reactions due in particular to the instability of these reagents.
2. Prior art
A method for decontaminating alkaline waste of caesium, based on the insolubility of caesium tetraphenylborate, was implemented in a single precipitation reactor with a long stay time in conjunction with a liquid-solid separation by tangential filtration.
However, the reaction is very difficult to control since tetraphenylborate is unstable and implementation of the method is accompanied by a strong release of benzene and other unstable aromatic compounds in the reactor, jeopardising the safety of the method. Moreover, the quantity of reagents used is high and is reflected in the cost of the decontamination process.
The purpose of the present invention is precisely to overcome the above-cited disadvantages by providing a method to separate caesium, strontium and tranuranium elements contained in sodium waste, which entails the use of TPBxe2x88x92.
The method of the invention comprises, in line, the following successive steps:
(a) filling at least one of two feed tanks with the waste,
(b) analysis of the content of caesium, strontium and other transuranium elements present in the waste,
(c) pre-treatment of the waste consisting of, in one of the feed tanks filled with the waste, mixing said waste with an insolubilising agent for strontium and transuranium elements in a quantity adapted in particular in relation to the analysis of step (b), mixing being conducted for a first stay time and at a first temperature that are adequate for insolubilising the strontium and transuranium elements, said pre-treated waste forming a first suspension.
(d) an optional separation step to separate, from the first suspension, the insolubilising agent bound to the strontium and the transuranium agents, and to obtain a first liquid phase, free of said insolubilising agent, containing the caesium and sodium in solution.
(e1) a first, continuous, caesium separation treatment conducted on the first suspension derived from step (c) or on the first liquid phase derived from step (d), consisting of:
firstly mixing, in a first precipitation reactor, said first suspension or said first liquid phase with TPBxe2x88x92 in appropriate quantity relative in particular to the analysis of step (b), and to a first measurement of the caesium content made at step (f1) downstream from this first separation treatment, mixing being conducted for a second stay time and at a second temperature adequate to precipitate the caesium, contained in the first suspension or in the first liquid phase, with the TPBxe2x88x92 and to limit degradation of the TPBxe2x88x92, so as to obtain a second suspension containing, in suspension in a second liquid phase, the insoluble TPBxe2x88x92 containing the TPBxe2x88x92 bound to the caesium, and if present the insolubilising agent bound to the strontium and transuranium elements, and
secondly, separating from the second suspension the insoluble TPBxe2x88x92 containing the TPBxe2x88x92 bound to the caesium, and if present the insolubilising agent bound to the strontium and transuranium elements, in order to obtain the second liquid phase, said first precipitation reactor being continuously supplied with TPBxe2x88x92 and the first suspension or the first liquid phase through the alternate use of the different feed tanks for said supply,
(f1) the first measurement of the caesium content remaining in said second liquid phase,
(g) recovery of the insoluble TPBxe2x88x92 containing the TPBxe2x88x92 bound to the caesium, and if present of the insolubilising agent bound to the strontium and transuranium elements derived from the preceding steps, and optionally,
(h) extraction treatment of the caesium bound to the recovered TPBxe2x88x92.
The method of the invention may also, between steps (f1) and (g), comprise the following steps:
(e1a) a second continuous separation treatment of the caesium conducted on the second liquid phase derived from step e1) consisting of:
firstly mixing, in a second precipitation reactor, said second liquid phase with TPBxe2x88x92 in appropriate quantity relative in particular to the first measurement of step f1) and to a second measurement of the caesium content made at step f1a) downstream from this second separation treatment, mixing being conducted for a third stay time and at a third temperature suitable for precipitating the caesium present in the second liquid phase with the TPBxe2x88x92 and for limiting degradation of the TPBxe2x88x92, such as to obtain a third suspension containing, in suspension in a third liquid phase, the TPBxe2x88x92 bound to the caesium, and
secondly, separating from the third suspension the insoluble TPBxe2x88x92 containing the TPBxe2x88x92 bound to the caesium in order to obtain said third liquid phase, said second precipitation reactor being continuously supplied with TPBxe2x88x92 and the with the second liquid phase derived from the first separation treatment,
(f1a) said second measurement of the caesium content in said third liquid phase.
TPBxe2x88x92, for example in the form of sodium tetra-phenylborate, may be used to extract caesium from a solution. However, this reagent is easily degraded into benzene and other aromatic products, in particular by radiolysis in contact with the radioactive elements present in the waste and by chemical reaction with various constituents of said waste. With the method of the present invention it is possible, among others, to better manage the caesium separation process using TPBxe2x88x92 than with the methods of the prior art, to control the quantity of TPBxe2x88x92 used and hence to limit benzene emissions.
If TPBxe2x88x92 is used in NaTPB form, persons skilled in the art will easily understand that the TPBxe2x88x92 -precipitate recovered at step (e1) and optionally at step (e1a) also contains NaTPB.
With the present invention, it is possible to conduct the continuous separation of caesium, in particular in solid CsTPB form, from sodium waste while ensuring the necessary safety guarantees for the treatment of radioactive solutions.
At step b) of the method of the invention, analysis may be carried out using any technique known to persons skilled in the art to determine the quantity of each of the above-cited elements, for example by means of elementary analysis such as emission spectrometry, or for example by xcex3 or xcex2 spectroscopy for caesium 137 (137Cs).
Preferably, the first analysis is made after homogenisation of the waste in the feed tank. Homogenisation may be conducted mechanically.
Pre-treatment of the sodium radioactive waste at step c) makes it possible to insolubilise the strontium and transuranium elements using a first insolubilising agent. This agent is preferably chemically stable in the presence of the sodium waste and radioelements it contains. It is added in sufficient quantity to fix the above-cited elements. This quantity is determined and adjusted in relation to the analysis made at step b).
According to the invention, the insolubilising agent may be chosen from among any agent which adsorbs and precipitates strontium and transuranium elements, such as for example a titanate, a zeolite, barium sulphate or a mixture thereof. This agent may for example be monosodium titanate.
This agent may also be a mixture of different insolubilising agents, for example an insolubilising agent for strontium and an insolubilising agent for the transuranium elements.
According to the invention, the first stay time may be chosen in relation to the composition of the waste to be pre-treated, and to the nature of the insolubilising agent. In general, when using monosodium titanate (MST) it is for example approximately 12 to 36 hours, for example 24 hours.
The first temperature must allow insolubilisation of the strontium and transuranium elements. It may for example be 15 to 50xc2x0 C.
The purpose of the, at least, two feed tanks is in particular to provide continuous supply to the first precipitation reactor of a homogenous feed of known composition. These tanks alternately carry out the two functions described above. In one of the tanks, for example, decontamination of the strontium and transuranium elements may be achieved by stirring a mixture of the waste to be decontaminated and monosodium titanate (MST) for a time that is sufficient for the residual content of soluble strontium and transuranium elements to be sufficiently low, for example {fraction (1/100)} of the initial content. Stirring may be mechanical. The other tank, in which pre-treatment has already been conducted, is used to supply the first precipitation reactor.
With optional step d), the insolubilising agent bound to the strontium and transuranium elements may be separated from the first suspension, derived from step c), to obtain firstly the insolubilising agent bound to the strontium and transuranium elements, and secondly a first liquid phase free of said insolubilising agent containing the caesium and sodium in solution.
This separation step may, for example, consist of applying to the pre-treated waste a conventional solid-liquid separation method. This step may be conducted in the feed tanks or between the feed tanks and the first separating unit. Step e1) of the method of the invention may then be conducted on the first liquid phase derived from this step d).
At step e1), the first precipitation reactor may for example be continuously stirred and supplied firstly with the first suspension derived from one of the feed tanks or with the first liquid phase derived from step d), and secondly with a continuous, controlled flow of TPBxe2x88x92.
The reactor, stay time and precipitation temperature of the caesium are discussed in further detail below.
At step e1), the separation of the insoluble TPBxe2x88x92 containing the TPBxe2x88x92 bound to the caesium, and if present of the insolubilising agent bound to the strontium and transuranium elements, from the second liquid phase may be made by filtration.
Therefore, according to the invention, at step e1), the first precipitation reactor may be associated with first continuous filtration means fitted with a first continuous evacuation outlet for the insoluble TPBxe2x88x92 containing the TPBxe2x88x92 bound to the caesium derived from the first separation treatment, and if present for the insolubilising agent bound to the strontium and transuranium elements derived from the pre-treatment step, and with a second outlet for the continuous evacuation of the second liquid phase derived from this first separation treatment. In this case, said second suspension is continuously added from the first precipitation reactor into said filtration means for the continuous separation and evacuation, through said first outlet, of the insoluble TPB containing the TPBxe2x88x92 bound to the caesium, and if present of the insolubilising agent bound to the strontium and transuranium elements, and of the second liquid phase derived from this first separation treatment through said second outlet.
The second suspension may, for example, be continuously evacuated from the precipitation reactor by overflow, falling under gravity into a chute which supplies the filtration means.
The method of the present invention may also comprise a washing step of the precipitate with water or buffered water in the filters before clearing. The washing waste derived from this step may follow the corresponding liquid phases.
The filtration means may be frontal filtering means, for a example a rotating drum filter operating under a vacuum for example.
According to the invention, at step f1), the first measurement may be a line measurement of the caesium content remaining in the second liquid phase after the first separation treatment. This measurement may for example be made using a xcex3 or xcex2 spectroscope for caesium 137. It is used, together with the analysis of step a), to adjust the quantity of TPBxe2x88x92 continuously added to the first precipitation reactor in order to limit excess of this reagent while optimising caesium precipitation.
The second liquid phase derived from the first separation treatment may be subjected to a second continuous, caesium separation treatment, for example if the sodium waste has a high caesium content and the first separation treatment does not lead to sufficient caesium separation. The first measurement used to control the caesium content in the second liquid phase derived from the first separation treatment may give an indication as to the utility of a second separation treatment.
According to a first variant of the method of the invention, the method may also comprise, between steps f1) and g), the steps e1a and f1a) described above. According to this first variant, the second separation treatment may be conducted in a precipitation reactor such as the one previously described.
At step e1a), the insoluble TPBxe2x88x92, containing the TPBxe2x88x92 bound to the caesium, may be separated by filtration.
Therefore, according to the invention, at step e1a) the second separation unit may be associated with second means of continuous filtration, fitted with a third outlet for the continuous evacuation of the insoluble TPBxe2x88x92 bound to the caesium derived from this second treatment, and with a fourth outlet for the evacuation of said third liquid phase derived from this second treatment. In this case, said third suspension may be continuously added to said filtration means to separate and evacuate in continuous manner, on one side via said third outlet the insoluble TPBxe2x88x92containing the TPBxe2x88x92 bound to the caesium, and on the other side, via said fourth outlet, said third liquid phase.
In short, the second liquid phase may continuously supply the second precipitation reactor, and the second precipitation reactor may be associated with filtration means functioning according to the same principle as the first.
It is possible to add a small quantity of potassium ions to the second precipitation reactor in order to improve the efficacy of TPBxe2x88x92 precipitation of the caesium.
According to a second variant of the method of the invention, distinct from the first, since the level of radioactivity is lower in the second liquid phase than in the first liquid phase, the method may also comprise, between steps f1) and g), as second separation treatment, a step e1b) to separate the caesium from the second liquid phase with an adsorption treatment on an appropriate ion exchanger such as a silicotitanate or a resorcinol resin. This step e1b), as in the first variant, leads to obtaining a third liquid phase. The caesium content is measured, using the means already indicated, on the third liquid phase to verify caesium decontamination of the solution.
The third liquid phase derived from the second separation treatment of the method of the invention, whether of the first variant or second variant, forms a fraction decontaminated of the caesium, strontium and transuranium elements in the initial waste.
According to the invention, the second and third stay times are preferably limited to the minimum stay time required in order to obtain the desired level of caesium separation, and to prevent too considerable degradation of the TPBxe2x88x92. For example, they may irrespectively range from 30 minutes to 4 hours, for example between 30 minutes and 2 hours.
According to the invention, the first and second precipitation reactors may be thermostat controlled to control treatment temperature. For example, the second temperature and the third temperature suitable for precipitating the caesium may irrespectively range from 20 to 50xc2x0 C.
According to the invention, at step f1a), the second measurement may be continuous measurement of the content of caesium remaining in the third liquid phase after the second treatment. It can be used in particular to adjust the quantity of TPBxe2x88x92 added to the second precipitation reactor as previously described. This measurement may be made as described above.
The crown of the first and second precipitation reactors and all the parts of the device for the implementation of this method in contact with TPBxe2x88x92, may swept by a flow of gas to prevent an accumulation of benzene. This gas may for example be nitrogen or air or a mixture of air and nitrogen, provided that the benzene concentration in the gas remains well below the flammability limit in this gas. If air is used, it may be supplemented by nitrogen.
According to the invention, the caesium being bound to the TPBxe2x88x92 when it is recovered at step g), it may be subjected to the extraction treatment of step h), said extraction treatment comprising for example destruction of the TPB ion. Destruction of the TPBxe2x88x92 ion is performed for example in a destroyer reactor for this ion with appropriate reagents such as formic acid and copper.
Therefore, according to one particular embodiment of the present invention, the precipitates derived from the first and optionally the second separation treatment may, for example, transit through a small-volume hydraulic guard and, by means of a rotary dispenser for example, then fall into one of two reactors for the destruction of the tetraphenylborate ion (TPBxe2x88x92). At the time when one of these destroyer reactors receives this mixture while being heated and supplied with appropriate reagents for the destruction of the TPBxe2x88x92 ion, the other reactor, already containing the mixture and the reagents is heated to exhaust the precipitate and solution of benzene and other degradation products of TPBxe2x88x92. Alternate operation of the two reactors allows for continuous destruction treatment. The benzene and other volatile degradation products derived from destruction of the tetraphenylborate ion may be carried off by steam; they may for example undergo counterflow washing in water, then be condensed and directed towards a benzene-destroying reactor, for example an incinerator.
The caesium, and if present the first insolubilising agent bound to the strontium and transuranium elements recovered after TPBxe2x88x92 destruction, could supply a vitrification facility. They may be vitrified for storage purposes as the majority of Na+ ions are carried away with the liquid phases.
The sodium waste may also contain potassium or other elements such as mercury. Since TPBxe2x88x92 fixes these elements also, it is necessary in this case to take into account their concentration in the waste to be treated in order to adapt the quantity of TPBxe2x88x92 at step e1), and in the first variant at step e1a). The first analysis can be used to determine the quantity of these elements and to adapt the separation treatment. The examples below illustrate this instance.